This invention relates to plasma density measuring method and apparatus for use in an apparatus for producing a plasma by introducing a high frequency electric power through a dielectric member.
Conventional plasma density measuring methods are mainly based on inserting a probe into a plasma.
As an example, Basics of Plasma Engineering (Akasaki, et al., Sangyo Tosho Shuppan, p207) describes a method of measuring plasma density and temperature by using a Langmuir probe. FIG. 1 shows a plasma density measuring apparatus using a Langmuir probe. In FIG. 1, denoted at 301 is an annular wave guide tube, and denoted at 302 is a slot antenna. Denoted at 303 is a dielectric member, and denoted at 304 is a vacuum container having a window as closed by the dielectric member 303. Denoted at 305 is a plasma, and denoted at 306 is a Langmuir probe. Denoted at 307 is an ammeter, and denoted at 308 is a voltage source. In this Langmuir probe 306, an electric voltage is applied to a probe as inserted into the plasma 305, and charged particles within the plasma 305 as flown into the probe are detected as an electric current. Thus, from the voltage dependency of the probe current, the electron density of the plasma as well as the electron temperature, for example, can be detected.
However, in the Langmuir probe 306, the free end of the probe for measuring the electric current is made of metal. This raises various inconveniences such as etching of the probe by a corrosive gas plasma, deposition of an insulating film on the probe surface due to a depositing gas plasma, introduction of metal contamination into a processing chamber, and disturbance of the plasma 305, for example.
As an attempt to solving these problems, a plasma absorbing probe was proposed as a contamination free probe, in 59th Autumn Academic Lecture 15p-C-17 of Japanese Association of Applied Physics, 1998. The structure and the principle of such probe are as follows. A dielectric material tube with its free end closed is inserted into a plasma, while a coaxial cable is inserted into the dielectric material tube. A high frequency wave is applied to the coaxial cable, and the frequency thereof is then changed. In response to it, a surface wave plasma is produced at the interface between the dielectric member and the plasma, with a frequency corresponding to the plasma density, and thus the absorption of a high frequency power occurs. By measuring the frequency with which the absorption occurs, the plasma density can be calculated. The relation between the plasma density ne and the absorption frequency fsw can be expressed by the following equation:
ne=∈0me(l+∈r)(2xcfx80fsw/e)2
With such a plasma absorbing probe, two of the inconveniences involved in the Langmuir probe, that is, probe etching due to the corrosive gas plasma and deposition of an insulating film on the probe surface by a depositing gas plasma, are improved significantly. Also, the introduction of metal contamination into the processing chamber can be prevented completely. However, the problem of plasma disturbance is still unsolved. Particularly, where probe inserted (in-situ) monitoring of a plasma for use in a production machine is to be executed, the insertion of the probe directly causes degradation of the uniformity of processing, and therefore, it is impractical. In consideration of this, a few proposals for measuring the state of a plasma without contact thereto have been proposed.
As an example of non-contact type plasma density measuring method, Japanese Laid-Open Patent Application, Laid-Open No. 256845/1992, discloses a microwave interference type plasma density measuring method. This method is based on the phenomenon that a microwave propagated through a plasma has a phase shift as compared with a microwave propagated through an atmosphere. The phase difference xcex94"PHgr" of an electromagnetic wave passed through a plasma has a relation with the plasma density net as follows:
xcex94"PHgr"=lxcfx89pe2/2cxcfx890
xcfx89pe2=e2ne/∈0me
where l is the length of the plasma to be passed through, c is the light speed, xcfx890 is the wavelength of the electromagnetic wave in a vacuum, e is the elementary charge, and ∈0 is the dielectric constant.
In this method, the produced microwave is divided into two paths. One of the paths passes through the plasma and, after that, it is re-combined with the other, whereby interference is produced. As regards the microwave passing through the plasma, the phase thereof changes in accordance with the dielectric constant thereof. Thus, when it is caused to interfere with the microwave passed through the atmosphere and by measuring the waveform thereof, the plasma dielectric constant can be detected on the basis of the phase change in the plasma. As a result, the plasma density can be obtained.
As a similar example, Japanese Lald-Open Patent Application, Laid-Open No. 128764/1994, shows a method using laser interference.
This is a completely non-contact method and thus it does not produce a plasma disturbance. However, the measuring apparatus becomes very large. Also, there is a restriction that a window must be provided at a position opposed to the chamber. Further, the adjustment of the microwave path is not easy. For these reasons, practically it is very difficult to incorporate this measuring unit into a production machine.
As described above, with conventional plasma density measuring methods using probes, at least the problem of plasma disturbance inevitably occurs.
Further, with conventional plasma density measuring methods using microwave interference, while the problem of disturbance can be avoided because it is non-contact to the plasma, there still remain inconveniences such as that the measuring system is very large. It is therefore practically very difficult to incorporate the unit into a production machine.
It is an object of the present invention to provide a plasma density measuring apparatus and/or a plasma density measuring method with which plasma disturbance can be avoided and the unit can be made compact and thus it can be easily incorporated into a production machine.
It is another object of the present invention to provide a plasma processing system and/or a plasma processing method, which uses the plasma density measuring method and apparatus as described above.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.